Method for manufacturing press-molded article, and press-molded article

Abstract

A method for manufacturing a press-formed article includes: heating a specific steel sheet for hot-pressing at 900° C. or more and 1,100° C. or less; starting press forming of the steel sheet which has been heated; and then, cooling the steel sheet to a temperature equal to or less than a temperature 100° C. below a bainite transformation starting temperature Bs and equal to or more than a martensite transformation starting temperature Ms, while ensuring an average cooling rate of 20° C./sec or more in a mold during forming and after the completion of forming; and further cooling the steel sheet, which has been cooled, to 200° C. or less at an average cooling rate of less than 20° C./sec.

1 . A method for manufacturing a press-formed article, the method comprising:
 heating a steel sheet for hot-pressing at 900° C. or more and 1,100° C. or less, to obtain a heated steel sheet;   beginning press forming of the heated steel sheet;   cooling the steel sheet to a temperature equal to or less than a temperature 100° C. below a bainite transformation starting temperature (Bs) and equal to or more than a martensite transformation starting temperature (Ms), while ensuring an average cooling rate of 20° C./sec or more in a mold during the press forming and after completion of the press forming to obtain a cooled steel sheet; and   further cooling the cooled steel sheet to 200° C. or less at the average cooling rate of less than 20° C./sec,   wherein   the steel sheet comprises
 C: from 0.15 to 0.5% by mass, 
 Si: from 0.2 to 3% by mass, 
 Mn: from 0.5 to 3% by mass, 
 P: 0.05% by mass or less (exclusive of 0%), 
 S: 0.05% by mass or less (exclusive of 0%), 
 Al: from 0.01 to 1% by mass, 
 B: from 0.0002 to 0.01% by mass. 
 Ti: equal to or more than 3.4[N]+0.01% by mass and equal to or less than 3.4[N]+0.1% by mass, wherein [N] indicates a content (mass %) of N, 
 N: from 0.001 to 0.01%, iron, and 
 unavoidable impurities; 
   an average equivalent-circle diameter of a Ti-containing precipitate having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the steel sheet is 6 nm or less; and   a precipitated Ti amount and a total Ti amount in a steel of the steel sheet satisfies formula (1):
   Precipitated Ti amount (mass %)−3.4[N]<0.5×[total Ti amount (mass %)−3.4[N]]  (1)
 
   
     
     
         2 . The method according to  claim 1 , wherein the steel sheet for hot-pressing further comprises, as the other element(s), at least one of the following (a) to (c):
 (a) at least one selected from the group consisting of V, Nb and Zr, in an amount of 0.1% or less (exclusive of 0%) in total;   (b) at least one selected from the group consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less (exclusive of 0%) in total; and   (c) at least one selected from the group consisting of Mg, Ca and REM, in an amount of 0.01% or less (exclusive of 0%) in total.   
     
     
         3 . A press-formed article of a steel sheet comprising:
 C: from 0.15 to 0.5% by mass;   Si: from 0.2 to 3% by mass;   Mn: from 0.5 to 3% by mass;   P: 0.05% by mass or less (exclusive of 0%);   S: 0.05% by mass or less (exclusive of 0%);   Al: from 0.01 to 1% by mass;   B: from 0.0002 to 0.01% by mass;   Ti: equal to or more than 3.4[N]+0.01% by mass and equal to or less than 3.4[N]+0.1% by mass, wherein [N] indicates a content (mass %) of N;   from 0.001 to 0.01%;   iron; and   unavoidable impurities,   wherein:   a metal microstructure of the press-formed article includes   bainitic ferrite: from 60 to 97 area %,   martensite: 37 area % or less,   retained austenite: from 3 to 20 area %, and   remainder microstructure: 5 area % or less;   an average equivalent-circle diameter of Ti-containing precipitate having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the press-formed article is 10 nm or less; and   a precipitated Ti amount and a total Ti amount in a steel of the steel sheet satisfies formula (1):
   Precipitated Ti amount (mass %)−3.4[N]<0.5×[total Ti amount (mass %)−3.4[N]]  (1)
 
   
     
     
         4 . A press-formed article of a steel sheet comprising:
 C: from 0.15 to 0.5% by mass;   Si: from 0.2 to 3% by mass;   Mn: from 0.5 to 3% by mass;   P: 0.05% by mass or less (exclusive of 0%);   S: 0.05% by mass or less (exclusive of 0%);   Al: from 0.01 to 1% by mass;   B: from 0.0002 to 0.01% by mass;   Ti: equal to or more than 3.4[N]+0.01% by mass and equal to or less than 3.4[N]+0.1% by mass, wherein [N] indicates a content (mass %) of N;   N: from 0.001 to 0.01%;   iron; and   unavoidable impurities,   wherein:   the steel sheet for hot-pressing further comprises, as the other element(s), at least one of the following (a) to (c):   (a) at least one selected from the group consisting of V, Nb and Zr, in an amount of 0.1% or less (exclusive of 0%) in total;   (b) at least one selected from the group consisting of Cu, Ni, Cr and Mo, in an amount of 1% or less (exclusive of 0%) in total; and   (c) at least one selected from the group consisting of Mg, Ca and REM, in an amount of 0.01% or less (exclusive of 0%) in total;   a metal microstructure of the press-formed article includes   bainitic ferrite: from 60 to 97 area %,   martensite: 37 area % or less,   retained austenite: from 3 to 20 area %, and   remainder microstructure: 5 area % or less;   an average equivalent-circle diameter of Ti-containing precipitate having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the press-formed article is 10 nm or less; and   a precipitated Ti amount and a total Ti amount in a steel of the steel sheet satisfies formula (1):
   Precipitated Ti amount (mass %)−34[N]<0.5×[total Ti amount (mass %)−3.4[N]]  (1).